The present invention relates to a cigarette with a dual-structure filter and, more particularly, to a cigarette with a dual-structure filter, having a high draw resistance and exhibiting a decreased CO/tar ratio in main stream smoke.
With recent changes in the taste of smokers, low-nicotine, low-tar cigarettes are being desired. Accordingly, a plurality of air holes (ventilation holes or vents) are formed in a so-called tip paper for connecting a tobacco rod and a filter along the circumferential direction of the filter. When a smoker inhales smoke, air flows into the filter through these holes to increase the air inflow rate (also called the filter ventilation rate) from the tip paper. When the air inflow rate increases, the air amount in main stream smoke increases, and the concentrations of nicotine and tar relatively decrease.
When the air inflow rate is increased, however, the suction resistance (product air-permeation resistance) of a cigarette product lowers. This lowers the draw resistance during smoking, so the original taste of the product cannot be maintained any longer.
Presently commercially available cigarettes with filters generally have a tar amount of 1 to 15 mg/cigarette and a CO/tar ratio (i.e., the weight ratio of carbon monoxide (CO) to tar contained in the main stream smoke) of 1 or more. Cigarettes having a CO/tar ratio of as high as 1.5 are also marketed. Recently, cigarettes having a CO/tar ratio of less than 1 are being desired.
One method of decreasing the CO/tar ratio is to use a filter having a low tar-filtering efficiency and at the same time increase the filter air-permeability. When a filter having a low tar-filtering efficiency is used, the tar amount in the main stream smoke increases to decrease the CO/tar ratio. When the filter air-permeability is increased, the combustion amount of a cigarette reduces to reduce the tar amount, but the CO/tar ratio does not change substantially. Therefore, the combination of the two can decrease the CO/tar ratio while maintaining the tar amount in the main stream smoke at a predetermined value.
Unfortunately, the just-described method also lowers the product air-permeation resistance of a cigarette.
Particularly, cigarettes having filter ventilation (i.e., having means, such as ventilation holes formed in a tip paper, for flowing air from the circumferential surface of a filter) are said to require a product air-permeation resistance of 90 to 130 mmH.sub.2 O in order to maintain a good taste. However, the product air-permeation resistance of cigarettes whose CO/tar ratio is decreased by the above method does not reach 90 mmH.sub.2 O. This deteriorates the taste of the cigarettes.
As another method of decreasing the CO/tar ratio, Jpn. Pat. Appln. KOKAI Publication No. 62-175162 has disclosed a filter using a special material such as a plastic film, e.g., a polyethylene film, as a filter element. Also, Jpn. Pat. Appln. KOKOKU Publication No. 4-16151 has proposed a filter having a special material and a special structure, such as a filter having a plastic tubular inside member whose tip is crimped. These filters can lower the CO/tar ratio. However, the use of the special materials and structures increases the manufacturing cost and makes the filters difficult to manufacture.